Highway tractor trailer vehicles include separate systems of brakes on each of the tractor and the trailer. The systems are connected when the tractor is hitched to the trailer by, connecting a set of air lines from the tractor to the trailer. The systems are coordinated such that when the brake pedal on the tractor is depressed, the brakes on both the tractor and the trailer are activated.
A separate manual hand operated trailer brake control or “spike” control is also provided in the track so that the operator can activate the trailer brakes without activating the truck brakes. The manual trailer brake control is used at low speeds to check that the trailer brakes are operating by allowing the truck to roll while applying only the trailer brakes.
The operator will know the trailer brakes are operating because the trailer brakes will stop the vehicle without any tractor brakes being applied. This procedure, sometimes called a “tug test” is a safety measure in that testing the trailer brakes immediately after connection of the tractor to the trailer also ensures that die trailer is properly connected to the tractor. If the connection is not secure, instead of the trailer dragging the tractor to a stop when the trailer brakes are applied, the connection will separate. Such separation at low speeds in the trucking yard poses a much safety risk compared to such separation later in the trip. Separation of the trailer from the tractor presents a significant safety risk, and stopping the truck by applying the trailer brakes, ensures that the connection is secure.
The manual trailer brake control is also necessary to allow the trailer brakes to be applied independently to control a jack-knife situation. Activating the trailer brakes alone slows the trailer down and pulls the vehicle straight to correct the jack-knife situation.
Brake failure or malfunction is a significant factor in highway accidents. Brake adjustment and function are checked by personnel at highway truck inspection points who have the power to pull a tractor or trailer out of service until the brakes are adjusted and functioning properly. Brake adjustment is time consuming, especially given the large number of wheels on tractor trailer vehicle. Self-adjusting brakes have therefore been developed for use on both tractors and trailers and operate to maintain proper adjustment of brakes and keep the vehicle on the road, however brake maintenance to correct wear on brakes remains a considerable expense for the owner of tractors and trailers.